It happened in my case before, when some diagonal matrix elements had too small value, e.g. 2.2204e-16. They make the whole matrix close to be "singular"; then it can not be properly diagonalized to solve the eigen problem of chi2 and fails to get a better fitted parameter in the current iteration.

If there are too many these messages, the resolutions are always far away from the expectation in my analysis due to poor kinematics.